1. Field of the Invention
The invention relates to a process for producing a chill roll having surface properties that are highly desirable for use in the hot rolling of steel. More particularly, the invention relates to the discovery that the introduction of niobium into a chilled-iron roll casting composition produces surface hardness values not previously attainable without interfering with the balance between carbide formation and free graphite dispersion that is necessary in such casting compositions.
2. Background of the Invention
In the continuous hot rolling of steel strip, a continuously moving steel workpiece (the strip) is passed through a rolling mill which commonly consists of several stands of rolls arranged in a straight line (in tandem). The strip cools as it passes through the rolling mill, such that each succeeding stand is at a lower temperature than its predecessor stand. Typically, when the strip reaches the rolls of the last few mill stands there is a tendency of the strip to weld or fuse to the rolls through which it passes because of the lower temperature of the roll. The results of such welding can be a catastrophic demolition of the rolling mill stands and surrounding structures, not to mention the grave threat to workers in the area.
It is evident, therefore, that the selection of the proper grade of roll to be used in the latter stands of tandem style rolling mills is important. The problem of roll selection is complicated by the fact that mill conditions vary widely, but in general the finishing rolls on a tandem hot mill should have an outer skin which is dense and hard, and yet provide sufficiently low friction in the areas that contact the workpiece.
Since the early days of steelmaking, rolling mill rolls have been cast in a manner to ensure that the liquid iron on the outer surface of the roll is cooled to produce the desired structure and properties. One technique for attaining this rapid cooling is to insert metal rings or segments, called "chills", in the mold, close to the surface to be contacted by the molten iron. The production of the chill roll shells typically involves a two step process, in which an outer shell in formed that possesses the aforementioned qualities necessary for use in a rolling mill followed by the formation of an inner core composed of a material that provides additional strength to the chill roll, such as cast iron. The outer shell is formed by either a static or spin pour, as is well known in the industry, an example of which is U.S. Pat. No. 5,355,932 issued to Nawata et al.
Most early chill rolls were cast using ordinary low silicon iron alloyed with nickel and chromium and chilled at a very high rate to suppress the formation of graphite, which was thought to be detrimental to the roll due to the softness imparted to the alloy by the graphite. The chilled outer surface is very hard and, when fractured, has a white fracture face for a distance beneath the surface (known as the chill zone), signifying that the formation of free graphite in that area had been suppressed by the rapid cooling. The white iron zone sometimes is referred to as "white cast iron", as contrasted with iron containing graphite that has a grey fracture face, known as "grey iron".
In the 1930s, it was discovered that the introduction of finely dispersed graphite into the white iron zone substantially reduced roll breakage despite providing for a softer outer shell. The region of the finely dispersed graphite in the alloy is termed "mottled." The presence of graphite in the outer shell greatly improves the ability of the roll to withstand the thermal shocks associated with hot rolling steel strip, reduces the friction between the roll and the strip thereby lowering the applied stress on the strip, and greatly reduces the potential for fusing of the strip to the roll. As a result, white cast iron chill rolls were largely superseded by a roll characterized by finely dispersed graphite near the outer surface of the roll and the lack of a definite chill zone. Such a roll has become known as an "indefinite chill" roll (or a "grain" roll).
While indefinite chill rolls significantly improve the durability of the roll over white cast chill rolls, the presence of graphite provides for a softer roll having a lower wear resistance and a shorter usable life between regrinds than the more highly alloyed rolls in the same finishing stands. Considerable efforts have been made worldwide to develop rolls which do not weld to the steel strip being rolled and have a better resistance to abrasion than the indefinite chill rolls. A primary focus of the efforts is on the use of metallic carbides to increase the hardness and abrasion resistance of an iron alloy as is known in the art; however, increasing the amount of carbides generally produces a commensurate reduction in the amount of graphite in the alloy. Numerous attempts have been made to develop alloys containing potent combinations of strong carbide forming elements, such as are used in tool steels, to replace the indefinite chill roll compositions. However, these high carbide, low graphite alloy rolls have also proven to be unsuitable for chill roll applications, because of the tendency to weld to the material being rolled and to initiate pressure cracks, much like the white cast iron chill rolls. For lack of a superior alternative, indefinite chill rolls have been retained in the late finishing stands of many of the modern high speed hot strip mills and the use of potent carbide forming elements has been limited to relatively small additions, usually of molybdenum, to indefinite chill roll compositions to alter the matrix structure or extremely small additions of magnesium to control the form of the graphite.
An essential feature of indefinite chill rolls is the critical balance between alloying elements such as carbon, nickel and silicon which promote the formation of graphite and carbide forming elements such as chromium. The formation of an alloy containing the proper balance of graphite and carbides requires extremely careful selection of melting stock, closely controlled melting conditions, rigid control of composition and inoculation techniques to obtain the required type and distribution of graphite. This relationship has inhibited the use of more potent carbide forming elements which greatly skew the graphite/carbide balance in favor of carbide formation and render the alloy unsuitable for use in indefinite chill roll applications. Thus, for over four decades the use of potent carbide forming alloys has been inhibited by the overwhelming need to maintain free graphite in the chilled structure of this type of roll.
One effort to improve the wear resistance of the chill roll material is presented in International Application Number PCT/GB93/02380 (the "'2380 application") published as International Publication Number WO 94/11541. The '2380 application discloses indefinite chill roll compositions produced by the introduction of solid carbide particles into a molten indefinite chill roll composition, and the subsequent solidification of the molten composition containing the solid carbide particles to produce a chill roll having encapsulated solid carbide particles.
As discussed in the '2380 application, both the methods of production and the resulting compositions encounter significant difficulties in material uniformity and carbide particle integration and elemental diffusion between the molten chill roll matrix and solid carbide particles introduced into the matrix. For example, coatings must be applied to the particles to help ensure adequate wetting of the particles by the molten chill roll matrix and proper solidification of the encapsulated particles in the matrix. Also, the composition of the coating material and the solid carbide particles and the introduction of the carbide particles must be precisely controlled to minimize elemental diffusion as a result of the nonequilibrium conditions between the solid carbide particles and the molten chill roll matrix. As such, the compositions and methods disclosed in the '2380 application do not provide a satisfactory solution to the problems associated with increasing the hardness and improving the wear resistance of indefinite chill roll structures without adversely affecting the desirable properties of the chill roll compositions.
Many other applications require the characteristics embodied in indefinite chill rolls, such as in plate mills, temper mills, narrow strip, backup rolls, bar mills for rolling flats, Steckel mills and a variety of cold temper mills. In all of these applications the present advantages of this type of roll would be greatly enhanced by a significant improvement in its resistance to abrasion.